[Click to enlarge] The new material stacks up favorably to kevlar and has plenty of room for even more improvement. (Source: ACS Nano/Northwestern University/Horacio Espinosa)

Mixture could lead to futuristic body armor, parachutes, and more

The
chief goal of industrial chemistry is to produce compounds with
useful characteristics at as low a price as possible. Researchers
from Northwestern University and various other institutions,
along with corporate partners, have certainly fulfilled the first
objective. They have created an exotic blend of polymer and the
ubiquitous nanomaterial, carbon nanotubes, that is stronger than
Kevlar.

The new material has extremely high
specific strength and energy-to-failure ratios. That
means that it can absorb a lot of impact without tearing, which in
turn means that the force of the impact will spread out rather than
be concentrated in a single destructive point. The yarn was
produced from double-walled carbon nanotubes and cross-linkable
organic polymers like polyvinylalcohol (PVA).

Interestingly,
the yarn fibers themselves had strength characteristics slightly
inferior to kevlar. For the engineers out there, the maximal
reported values for ductility was ~20%, ~100Jg-1 for
the energy to failure ratios, and ~1.4GPa for the specific
strength. But when the fiber bundles were woven together,
forming a macro-fiber with specific strength of ~6 GPa and energy to
failure ratios of ~500Jg-1.

These
materials of the woven cloth compared favorably to the 3 GPa strength
of kevlar and its approximately 30 Jg-1 energy
to failure ratio. In other words, that's good enough to
substantiate the University's claim that the material "could be
tougher than Kevlar", at least to some extent.

Heat
treatment is one key to increasing the energy to failure ratio,
likely because it activates the polymer cross-linking.

Engineering
professor Horacio
Espinosa led the study. He states,
"We want to create new-generation fibers that exhibit both
superior strength and toughness. A big issue in engineering
fibers is that they are either strong or ductile — we want a fiber
that is both. The fibers we fabricated show very high ductility and a
very high toughness. They can absorb and dissipate large amounts of
energy before failure. We also observed that the strength of the
material stays very, very high, which has not been shown before.
These fibers can be used for a wide variety of defense and aerospace
applications."

The research was significant as past
strength tests focused on pure-nanotube mixtures, which had
the tendency for the tubes to slip laterally when stressed, weakening
the resulting material. The new composite material is much
stronger as the polymers fix the nanotubes in place.

The
research was funded by a $7.5M USD grant from the Army Research
Office to investigate new materials for next
generation bulletproof vests, parachutes, or composite materials
used in vehicles, airplanes and satellites. The project is part
of the Department of Defense’s Multidisciplinary University
Research Initiative (MURI) program, which promotes defense-minded
projects that require a variety of engineering fields to
collaborate.

While
future work will focus in part on refining the material strength even
further, the crucial next step will be refining methods to mass
produce the fibers. A great
deal of work has already been put into mass producing carbon
nanotubes, so commercial super-strong carbon-polymer-based body armor
may not be as far off as one might think.

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